The first phase of the Marmaray project opened with an undersea rail tunnel (train pictured) across the Bosphorus strait.

Marmaray is a 76.6-kilometre-long (47.6 mi) commuter rail line located in Istanbul, Turkey. The line runs from Halkalı, on the European side, to Gebze, on the Asian side, along the north shore of the Sea of Marmara. Mostly using the right-of-way of two existing commuter rail lines, the Marmaray line linked the two lines via a tunnel under the Bosporus strait, becoming the first standard gauge rail connection between Europe and Asia. The two existing sections of the line were rebuilt and expanded from two tracks to three tracks, to allow for higher capacity with intercity and freight rail. The name Marmaray is a portmanteau of the words Marmara and Ray, which is Turkish for rail.

The Marmaray Tunnel is a 13.5 km (8.4 mi) long undersea railway tunnel in Istanbul, Turkey, beneath the Bosporus strait, linking Kazlıçeşme, Zeytinburnu in Europe with Ayrılıkçeşmesi in Asia. The tunnel consists of two single track tunnels with three underground railway stations: Yenikapı, Sirkeci and Üsküdar.

The Bosporus or Bosphorus Strait is a natural strait and an internationally significant waterway located in Turkey which is straddled by the city of Istanbul. The Bosporus connects the Black Sea to the Sea of Marmara and forms one of the continental boundaries between Asia and Europe. It also divides Turkey by separating Asia Minor from Thrace. It is the world's narrowest strait used for international navigation.